The present invention relates to an exhaust gas recirculation system for internal combustion engines, and more particularly to an exhaust gas recirculation system in which the flow quantity of exhaust gas into an intake manifold of the engine is controlled in accordance with operating conditions of the engine to reduce the nitrogen oxide content of exhaust emissions (hereinafter simply called NO.sub.x emissions).
An exhaust gas recirculation system has been proposed of the type which comprises a pneumatically operated servomotor to be operated by negative pressure applied thereto from an intake manifold of the engine, a flow control valve associated with the servomotor to control the flow quantity of exhaust gas through a recirculation pipe in response to operation of the servomotor, and a pair of electrically operated valves for selectively applying the negative pressure and the atmospheric pressure respectively from the intake manifold and the exterior to the servomotor. In such an exhaust gas recirculation system as described above, it is required to control the opening degree of the flow control valve in accordance with operating conditions of the engine as precisely as possible so as to effect recirculation of the exhaust gas in an optimum quantity. Additionally, in use of the system for a long period of time, there will occur clogging or obstruction in the recirculation pipe and/or the flow control valve assembly caused by adherence of carbon and lead contents or the like in the exhaust gas, resulting in decrease of the exhaust gas recirculation rate. It is, therefore, required to compensate the opening degree of the flow control valve in accordance with the extent of the clogging or obstruction so as to ensure recirculation of the exhaust gas in the optimum quantity.